The Satellite Digital Audio Radio System (SDARS) provides satellite antennae and terrestrial repeaters to transmit digital data frames of audio data over the air, and in the form of radio programs for listening by consumers. SDARS data frames are comprised of encoded (i.e. compressed) digital audio data that form into digital audio streams. The encoded audio streams are grouped with one another in data clusters, encoded with an error protection protocol, modulated to channels having appropriate carrier band frequencies and broadcast over the air for listening via channel tunable receivers.
An SDARS compatible radio receiver receives the satellite signal transmission, which is demodulated according to a channel selection that is made by the consumer at the receiver to decode a corresponding digital audio stream for listening. Two types of audio encoding are used.
A system of variable bit rate audio encoding for high quality music reproduction includes but is not limited to Perceptual Audio Coder (PAC), MP3, WMA or AAC. In the system audio compression algorithms are processed to attain high quality audio reproduction.
A system of fixed-bit-rate audio encoding is primarily suitable for audio reproduction of talk radio broadcasts, in which music reproduction is of low quality and of secondary importance. Such a system includes but is not limited to Multimode Transform Predictive Coder (MTPC), G.726, G.729, GH.723, GSM-HR, GSM-EFR or GSM-FR.
SDARS compatible radio receivers are especially suitable for use in vehicles, which travel beyond the effective range of terrestrial radio broadcasts. In vehicles, the listeners of talk radio satellite broadcasts may adjust their radio volume controls with reasonably high audio gain to enable understanding of spoken words over vehicular traffic noise and other environmental noise. When radio audience applause is reproduced at such reasonably high audio gain, the listeners are often compelled to manually turn down the radio volume controls until loud applause abates, and the quieter spoken words resume. Making such adjustments of volume controls distracts attention from the task of driving the vehicles.